This invention relates to all-fiber Mach-Zehnder interferometers and a method of making the same. More particularly, the invention relates to single-mode, fused optical fiber couplers that are concatenated to produce interferometric paths between the couplers, thereby resulting in a Mach-Zehnder structure. The invention also includes mechanical stabilization of such structure and its packaging.
All-fiber Mach-Zehnder interferometers generally comprise two fiber optic couplers fused in series on two parallel single-mode optical fibers. This basic structure is well described in the literature and in patents, such as U.S. Pat. Nos. 5,044,715 and 5,119,453. The all-fiber Mach-Zehnder appellation derives from the analogy between the classical two-path Mach-Zehnder interferometer and the fiber structure. A classical Mach-Zehnder interferometer is composed of two beam splitters and mirrors. A first beam splitter is used to split an input collimated light beam into two beams of equal intensity. These two beams are then redirected with mirrors at 45xc2x0 incidence on opposite sides of the second beam splitter, so that the reflexion of any one of the two beams will exactly coincide with the transmitted portion of the other beam. Because of this coincidence, the two beams interfere constructively or destructively at the output beam splitter, depending on the phase difference between the amplitude of the light in the two beams. This phase difference depends on the optical path length difference in the two beams, and the Mach-Zehnder output power in one beam is a sine square function of the phase difference and a complementary cosine square for the second output beam. If the beam splitters are exactly 50%, then the power transfer from one beam to the other is 100%. This is called the Mach-Zehnder effect.
In the all-fiber version, fused couplers are equivalent to beam splitters. The two paths correspond to the optical fibers between the couplers and the two output ports of the second coupler correspond to the two output beams. The fused couplers are made by laterally fusing two single-mode optical fibers together by applying a heat source on two optical fibers which are longitudinally in contact. The heated structure is then pulled, creating a bi-tapered structure. In this tapering, the light escapes the single-mode core, which becomes too small to guide the light and excites a superposition of two cladding modes, one symmetric and the other asymmetric. These modes have different propagation constants and will accumulate a phase difference. Because of the transverse modal intensity profile, the total transverse optical field, resulting from the interference between the two modes, will show a concentration of power varying with the phase difference between the two modes that will shift from one side of the fused fiber structure to the other. At the output taper of the coupler, this power distribution will result in the coupling of the output power in one fiber or the other, thus creating an exchange of power between the two output fibers. The ratio of power transferred is called the coupling ratio and because of the symmetry of the coupler, this ratio can vary from 0 to 100%, 50% or 3 dB being the case where the power split between the fibers is equal. In a Mach-Zehnder structure this is used to split light between two fibers, and then recombine and interfere the light at the output. This interference will depend on the optical path difference in the lengths of the two fibers between the two couplers.
U.S. Pat. No. 5,119,453 describes a Mach-Zehnder interferometer with small path differences. However, Mach-Zehnder interferometers with large path differences can also be made and have applications such as multiplexing and demultiplexing wavelengths onto and from a single-mode fiber. In these applications, the Mach-Zehnder structure transmission varies sinusoidally with wavelength, with a maximum amplitude. Depending on the wavelength, the power can output completely in one or the other of the output fibers.
The principle of an all-fiber Mach-Zehnder interferometer is fairly straightforward for one versed in the art, but as an interferometer, the Mach-Zehnder is very sensitive to any perturbation in its structure, such as mechanical changes or temperature changes. Furthermore, the greater the path difference, the smaller the wavelength period, thus making the selectivity of the wavelength response sharper, but at the same time the structure becomes more sensitive to outside environmental influences. The latter will cause the Mach-Zehnder sinusoidal wavelength response to shift, rendering the structure very good for sensing applications, but unusable in reliable wavelength multiplexing and demultiplexing applications. The difficulty in creating a satisfactory Mach-Zehnder structure thus resides both in the fabrication and in the packaging of the structure.
In dense wavelength multiplexing or demultiplexing applications, great care must be taken to achieve wavelength accuracy in the spectral response of the Mach-Zehnder with the maximum contrast. The couplers must be accurately fabricated to obtain a 50% coupling ratio at the middle wavelength in the wavelength range of operation, to insure the largest contrast possible and the fiber length difference between the couplers must be accurately controlled to insure the correct spectral response. Both these characteristics are difficult to achieve simultaneously.
As an example, one can easily fabricate two 3 dB fused couplers with exactly the correct spectral properties, and subsequently by fusing splice the output fibers of the first coupler to the input fibers of the second coupler. In such a case, however, it is extremely difficult to obtain the correct length difference which must be controlled within a micron to achieve the appropriate spectral response. Furthermore, splicing requires fiber length and the longer the fibers between the couplers, the more sensitive the structure is to external environmental factors such as temperature and vibrations. There is thus a need to produce all-fiber Mach-Zehnder interferometers that would obviate these disadvantages.
It is an object of the present invention to fabricate improved all-fiber Mach-Zehnder interferometers that obviate the disadvantages mentioned above.
Another object of the invention is to provide smaller and mechanically more stable Mach-Zehnder structures.
A still further object of the invention is to achieve better control in the fabrication and packaging of all-fiber Mach-Zehnder devices.
Other objects and advantages of the invention will become apparent from the following descriptions thereof.
In essence, the present invention provides an all-fiber Mach-Zehnder interferometer that comprises two fiber optic couplers made in series by fusing and tapering two parallel single-mode fibers and a central fiber structure between the two fiber optic couplers, said central fiber structure having two fibers of a shape that produces a predetermined path difference required for achieving a desired Mach-Zehnder effect, and is characterized in that the fibers of said central fiber structure are bonded in predetermined spots so as to stabilize the central fiber structure within the interferometer.
Thus, the invention involves creating a fiber structure by bonding two single-mode optical fibers in a given shape adapted to produce a predetermined optical path difference, and thereafter making two fused fiber optic couplers on the input and output fibers of such central fiber structure, so as to produce an all-fiber Mach-Zehnder interferometer. The two single-mode optical fibers may be bonded to each other or to other supporting fibers or to a substrate in order to retain the given shape of the central fiber structure unmodified, while the couplers are made and thereafter while the interferometer is packaged. Also, the central fiber structure is preferably so created that the input and output fibers coming out of the structure are parallel to each other and can easily be placed in longitudinal contact with each other so that they may be fused together and couplers may then be made in the fused sections by pulling and tapering the same.
Once the shape of the central fiber structure has been determined, the fibers can be formed into such shape by physical or mechanical means and bonded in such shape to a substrate. The two couplers can be made subsequently at the input and output ends of the central fiber structure, to form the Mach-Zehnder interferometer. The packaging of the couplers can then be completed on another substrate. However, it should be mentioned that if the central fiber structure is maintained on a different substrate than the couplers, this may result in some instability of the Mach-Zehnder interferometer and it is, therefore, preferable to place the entire interferometer arrangement including the central fiber structure and the couplers on a single substrate. Thus, the present invention also provides for bonding the Mach-Zehnder couplers to the same substrate that holds the fiber structure. This, however, requires that the couplers be made with the substrate already in place, which means that the fusing and tapering of the coupling regions must be carried out close to the substrate. This can be facilitated by designing the substrate so that it has notches or similar cavities in the area where the fibers are to be fused and tapered.
Moreover, in order to avoid the use of two substrates or a specially designed substrate, a further embodiment of the present invention provides for not using any substrate at all to hold the central fiber structure or the couplers prior to mounting the entire combination onto a packaging substrate. This is achieved by aligning the input fibers and the output fibers coming out of the central fiber structure approximately on the same axis. By so doing, the fabrication process of the couplers becomes more accurate and the packaging of the Mach-Zehnder interferometer becomes straight forward since everything is aligned on the same axis, making the packaging of the interferometer as simple as that of individual couplers. When everything is so aligned the couplers and the central fiber structure stabilized in accordance with the present invention can be held suspended in air in an approximately straight line, just by holding the input and output fibers at each end. A substrate is then approached to this overall arrangement suspended in the air and the entire Mach-Zehnder device is bonded thereto. In this manner, the couplers and the fiber structure can be bonded on a single substrate, which can then be packaged in a tube. The packaging can be done so that the input fibers of the Mach-Zehnder interferometer exit from one end of the tube and the output fibers from the other end of the tube.
For balanced or quasi-balanced Mach-Zehnder interferometers, keeping the two couplers on the same axis when forming the central fiber structure, is fairly straightforward. With only a few microns path difference, the two fibers of the structure are almost parallel in such cases. The natural shape of a structure composed of two fibers of different lengths, placed on the same geometric plane, is a crescent, where the shorter fiber forms the inner circular arc and the longer fiber forms the outer circular arc. These arcs are not perfectly circular because to make a coupler the fibers at the extremities of the crescent must be tangent. It is a feature of the present invention to bond the fibers at the points where they are tangent to each other, and the bond points can be between the fibers themselves without involving a substrate. For such structures, this is usually sufficient to provide the required mechanical stability.
When the path difference is very small, the crescent produced has a closed shape. If, under such conditions, the shorter fiber is pulled to a straight line position, the longer fiber will take the form of a bump in the linear arrangement of the two fibers, in order to maintain the same path difference. Such bump is not a natural shape of the fiber structure and, due to the mechanical rigidity of the fiber, some force must be exerted to maintain this unnatural shape of a bump in the fiber. For a small path difference, the force to be exerted is rather small and thus the pulling force exerted to make the couplers would normally be sufficient to keep the structure straight enough to make the Mach-Zehnder interferometer as this is disclosed in U.S. Pat. No. 5,044,715. Consequently, under these circumstances it is possible to make the couplers one after the other, after the fiber structure has been produced. This is suitable for very small path differences of few microns or few tents of microns, because the straightness of the fibers can be naturally maintained in such fibers.
However, for dense WDM multiplexers the path difference is usually several hundred microns and for very dense WDM multiplexers it is usually a few millimeters. In such cases it becomes much more difficult to control the plane of the bumps of the fiber, because the bump shape exerts a far stronger force. To satisfy the requirements of such dense multiplexers, a further embodiment of the present invention provides for positioning the long fiber within the central fiber structure in the form of an S that crosses the shorter fiber in the middle. The fibers are then bonded at the output tangent points and in the middle crosspoint. This produces a mechanically stable structure and greatly reduces any twisting of the S shape with reference to the plane of the output fibers. Due to this stability, the fiber structure can be easily manipulated so that the Mach-Zehnder couplers can be made on approximately the same axis. Furthermore, it is preferable to further bond the fibers at each output end of the central fiber structure, at points slightly further away from the primary bond points, after forcing the fiber into an accurate shape, so that the output transverse plane of the fibers is rotated a given angle. In a further embodiment, it is preferable to make the transverse planes at both output extremities parallel to each other and furthermore to have them parallel and on the same axis. This technique of parallel planes at both output extremities is particularly useful for very large path differences, to correct the angle and alignment that a simple S-shape cannot correct.
The S-shape of the fiber structure also addresses another problem which is the problem of sensitivity of the interferometer to vibration. One of the well known problems of a Mach-Zehnder structure is that if one of the fibers is under tension and the other is not, the all-fiber interferometer becomes sensitive to vibration or acoustic waves, making it unusable as a stable WDM device. This is the case of the bump-shape structure where the short fiber is held in a straight position. However, in the case of the S-shape structure, the two fibers are connected not only at the extremities, but also in the middle, making them less susceptible to different acoustic waves and to vibration. Furthermore, using the S-shape, the structure can be compressed and, being under very little external tension, it will keep its shape due to its internal tension and will not be sensitive to small acoustic perturbations. Thus, the S-shape structure can be bonded to a substrate under little tension or compression, making it acoustically insensitive.
The formation of the S-shape structure can be accomplished in several ways. For example, this can be done by applying approximate mechanical holders to the fibers and, while the fibers are held in place, applying bonding points in one or several steps. Also, a mold can be used that holds the fibers in an S-shape configuration while the bond points are applied. It is also possible to bond the fiber structure to the mold itself which would then act as a supporting substrate.
The present invention is not limited to the specific shapes of the structure mentioned above, such as the bump-shape or the S-shape, but covers any shape that will have the desired stability and a predetermined optical path difference. Any suitable method for achieving such structure is included within the scope of this invention. For example, the fibers need not be fully stripped of their protective jacket when the fiber structure is made; they can be stripped only at the bond points of the fiber suture. Also, it is possible to remove the protective polymer jacket from the fiber structure only after fabrication of the couplers; the protective jacket is not mechanically stable and, if left, may cause the interferometer to vary unpredictably as environmental parameters change. The Mach-Zehnder couplers can be bonded to separate substrates before bonding the whole structure to a final substrate.